If $4a + 9b + c = 1$ and $6x + 9y = 10$, what is $-5c + 12x - 45b - 20a + 18y$ ?
Explanation: $= -20a - 45b - 5c + 12x + 18y$ $= (-5) \cdot (4a + 9b + c) + (2) \cdot (6x + 9y)$ $= (-5) \cdot (1) + (2) \cdot (10)$ $= -5 + 20$ $= 15$